Supported chromium compounds have long been known as catalysts for the polymerization of .alpha.-olefins (see, for example, M. P. Mc Daniel, Adv. Cat 33, (1985) 47-98). The preparation of such supported chromium catalysts is usually carried out in two stages. In a first step, the support material is first brought into contact with a soluble chromium compound in a suitable solvent. Subsequently, in a second step, the mixture of support and chromium compound is calcined in a stream of air or oxygen at high temperatures, for instance from 400 to 1100.degree. C. (see, for example, U.S. Pat. No. 5,363,915).
It is known that the chromium catalysts can very easily be inactivated by catalyst poisons during the polymerization reaction. In EP-A-0 640 625, for example, states that it is extremely important to thoroughly remove such catalyst poisons such as moisture, oxygen, carbon monoxide, carbon dioxide and acetylene from the polymerization gas and that, for example, 2 ppm of carbon monoxide are sufficient to dramatically impair the polymerization.
The catalyst and polymer properties are influenced significantly by the composition of the support, the support structure (pores volume, mean pore radius, etc.), the calcination temperature and the chromium content. High polymer molecular weights can be achieved, for example, by a lower calcination temperature, although an often undesired broadening of the molecular weight distribution results.